The photovoltaic (PV) industry continues and is projected to have significant growth amidst the environment of fossil fuel supply uncertainties, concerns over global warming, difficulties of energy transport from distant centralized facilities, gradual yet steady improvements in PV module prices, re-evaluation of true costs for alternate energy in decentralized locations and other such factors.
Silicon is the main material resource for making solar cells and will remain so for a long time. The projected growth of the PV industry is singularly hampered by the lack of silicon feedstock material for the PV industry. While dedicated solar grade polysilicon plants are currently in production, or under construction, significant shortfall of silicon feedstock material is still expected in the foreseeable future. Many of the solar grade silicon manufacturing processes currently in use produce very significant quantities of high purity silicon powder as a by-product. However, despite the high purity of the silicon powder, this silicon powder by-product is presently very difficult to use, as will hereinafter be discussed in further detail.
More particularly, ultra fine silicon powder is currently a by-product of two major processes.
(1) the Fluid Bed Process to Manufacture High Purity Electronic or PV Grade Polysilicon.
In this process, silicon is deposited by heterogeneous thermal decomposition of silane (SiH4) gas or chlorosilane (SiClxHy, where y=4−x) gas on granules of silicon seed particles. The granules grow in size from an initial seed size of ˜0.2 mm to, typically, 1 mm in diameter. The granules are then utilized in silicon melting and crystal growth applications.
As stated above, the fluid bed process also results in the production of a large quantity of ultra fine silicon powder (or dust). More particularly, of the total amount of material produced by the fluid bed process, approximately 15% to 20% is ultra fine silicon powder. The particle sizes of such ultra fine silicon powder vary over wide ranges, but are typically in the range of approximately 0.1 microns to 20 microns in diameter. This powder is of high purity, but cannot currently be recycled or used in silicon melting and crystal growth applications because of the fineness of the powder.
More particularly, the powder produced by the fluid bed process is so fine that it becomes extremely difficult to handle. Among other things, the very fine silicon powder goes airborne easily, making it extremely difficult to transfer from location to location and making it hard to create “clean” (i.e., particle free) and inert atmosphere conditions by vacuum processes. In addition, the fineness of the powder creates significant maintenance problems for equipment. Furthermore, the fineness of the powder creates safety issues, since the powder can be explosive, in much the same way that corn dust may be explosive in a silo.
Accordingly, the very fine silicon powder created by the fluid bed process is tapped out of the reactor outlet as process waste.
(2) Ultra Fine Silicon Powder is the Primary Product from Free Space Reactors which are being Developed to Manufacture High Purity Electronic or PV Grade Polysilicon.
In this process, silicon powder is formed in the gas phase by homogeneous decomposition of silane (SiH4) gas or trichlorosilane (SiHCl3) gas at high temperatures. The particle size of such formed silicon powder is typically in the sub-microns range. The purity of the silicon powder produced by the free space reactor process is very high when used with high purity electronic or PV grade silane or trichlorosilane gas and when performed in suitable reactors. However, the ultra fine nature of the silicon powder prevents its direct use in silicon melting and crystal growth applications. This is because the fineness of the silicon powder creates the same transfer, “clean” and inert atmosphere, maintenance and safety issues discussed above with respect to the fluid bed process. Since the entire silicon production from the free space reactor process is in the form of such silicon powder, it needs to be processed and converted into larger forms for use by the PV industry.
A few methods have been attempted to convert ultra fine silicon powder (e.g., such as that produced by the two processes recited above) to larger forms so that they can be productively used as feedstock in manufacturing operations (e.g., such as silicon melting and crystal growth applications).
Granulation and augmentation of the silicon powder particle size by electron beam melting or microwave heating to 1200° C. to 1500° C. are described in Japanese Patent 11199382JP. Such procedures are mainly for making silicon nitride and other silicon compounds and are not applicable to the high purity silicon granules needed for the PV industry.
Typical powder metallurgical process schemes have been suggested to convert silicon powder into larger aggregates to make them useable in various applications. They include compacting with a typical organic binding agent such as starch and lignin.
High-pressure densification and hot pressing of silicon powder, with or without sintering aids, are described in J. Materials Sci. 31 (18), 4985-4990 (1996); J. Jap. Soc. Powder Metal. 28 (1)15-19 (1981); U.S. Pat. No. 4,040,848, and U.S. Pat. No. 4,040,849.
Dry compaction of high purity silicon powder without the use of binders is described in U.S. Pat. No. 7,175,685, but the compacted material does not have the strength and integrity needed for subsequent uses.
Compaction, using selected high purity inorganic or organic binders, with subsequent debinding and densification by high temperature sintering of pressed silicon bodies, in order to realize an industrially viable process and robust product, is described in U.S. patent application Ser. No. 11/479,735 (U.S. Patent Application Publication No. 2007/0014682).
Several procedures to make crystalline silicon spheres are described in various literatures. These procedures include: shotting of molten silicon through a nozzle or orifice as described in Asia Electronics Industry, p 45, February 2003, IEEE Vol. 31, p 963 (2005) and Jap. J. Appl. Phys. Vol. 46, p 5695 (2007); processing a paste of silicon deposited on a substrate and furnace melting the paste as described in U.S. Pat. No. 4,637,855; coating the silicon particles with an oxide skin layer, melting and coalescing the silicon into spheres within the oxide skin and further processes as described in U.S. Pat. Nos. 4,425,408, 5,069,740, 5,431,127 and 5,556,791, and other variations of such techniques as described in U.S. Pat. No. 5,817,173. Such silicon spheres, intended for spheral solar cells, are in the sub-millimeter to one millimeter diameter range. These processes are typically followed by some sequence of other processes to remove contaminations and cause crystallization, all of which make them inherently complicated for direct application.
Where it becomes necessary to maintain or improve the level of purity of the silicon powder for PV feedstock usage, application of conventional processes such as direct melting, powder compaction, sintering and densification, etc., or methods to form silicon spheres, create considerable complexity and associated costs, and in practice result in incorporation of unwanted impurities into the silicon.
In spite of the various above-mentioned proposals, there are currently no prior art industrially-practical and cost-effective methodologies available to directly convert silicon powders to high density granular forms which (i) maintain the purity and quality of the polysilicon powder by-product, (ii) are reproducible in a large-scale manufacturing environment, and (iii) may be transported and used without form failure and without the need for additional processes for subsequent product uses.
Accordingly, it is an object of the present invention to provide a robust process for the conversion of high purity silicon powder to high value densified granular forms which can be directly used by the PV feedstock industries to grow silicon crystals.
Laser-based processes are currently being developed in the realm of rapid tooling and rapid prototyping for use with metal powders. These processes include laser-assisted metal processes (LAMP), laser forming processes (LASFORM), solid freeform fabrication processes (SFF), shape deposition manufacturing processes (SDM), selective laser sintering processes (SLS) and laser engineered net shaping processes (LENS). Both CO2 and Nd-YAG lasers are utilized for such applications. A variety of metal powders, such as stainless steel, bronze, titanium, aluminum, copper and INVAR have been processed to make from small to complex net-shaped articles for industrial use. Additionally, high power laser systems are also being utilized for metal forming operations such as cutting, drilling, welding, micromachining, etc. Many such laser systems concentrate power through fiber optic and other focusing aids. High power solid state lasers are utilized for several such applications. They can be used on a continuous wave (CW), quasi-CW or pulsed mode depending on the application.
However, it is believed that, heretofore, no one has considered using lasers to convert high purity silicon powder to silicon granules and thereby to render this powder into viable feedstock.
Elemental silicon is uniquely positioned to utilize laser processing schemes. The optical absorption of elemental silicon increases exponentially when the incident radiation has wavelengths shorter than one (1) micrometer. This is illustrated in FIG. 1, where the optical absorption coefficient of silicon is plotted against the incident wavelength of the radiation.
The optical data for selected incident radiations are shown in Table 1. The optical absorption of silicon powder is expected to be a little higher than that of a polished wafer. Efficient absorption of short wavelength radiation will cause the silicon to heat and finally melt. See Table 1.
Several existing types of lasers have lasing wavelengths that can be applicable for processing silicon. These laser types include, for example, optically-pumped lasers of wavelengths slightly over 1 μm based on matrices of yttrium aluminum garnet (Nd:YAG, Yb:YAG), yttrium orthvanadate (Nd:YVO4) and lithium yttrium fluoride (Nd:LiYF4). However, the efficiencies of these lasers are below 30%. Because of their low efficiencies, their long wavelengths where silicon absorption is poor, and their high cost, these lasers are not ideal for melting of silicon powder on an industrial scale.
Diode lasers are also becoming more feasible for silicon processing, with larger power lasers becoming available. At the near infrared lasing wavelengths, very high power conversion efficiencies of greater than 65% have been achieved, since these devices are directly energized and electrically activated, rather than being optically pumped. The diode lasers utilizing gallium-arsenide (GaAs) and indium gallium arsenide (InGaAs), in the 940 nm to 980 nm range, which are used for optically pumping Yb:YAG or Yb:glass lasers, would themselves be highly efficient energy sources for silicon heating and melting.